Recently, plastic products have been widely used in various fields, and their utilities have a trend to increase. On the other hand, small pieces of waste plastic products become admixed in pulp and paper stuff or cereals and have accordingly caused a hindrance to processing the pulp and paper stuff or cereals. For example, if small pieces of plastic bags, braids, bands, tags, name plates, receptacles, toys or other plastic or rubber products (which will be hereinafter called "plastics") are admixed in wood chips and such wood chips are pulped as they are, the plastics will be contained in a paper web layer, with the result that pores are formed in the web layer or the web layer is easy to break. Such troubles will occur also when the plastics are admixed in straw, bagasse, hemp, rag, cotton, linter, or waste paper of non-wood pulp and papermaking materials.
Since the plastics are in general chemically stable, the plastics are subjected only to thermal deformation even in a liquor treatment at a high temperature and pressure in a pulping step. As the plastics are further pulverized into fine pieces by mechanical action of a beater, it is extremely difficult to remove the plastics from a pulp slurry by means of a screen of conventional type. As the difference in the specific gravity between the pulps and the plastics is slight, the plastics removal efficiency is low for the large power consumed by a cleaner which utilizes a centrifugal force. Consequently, when the plastics are admixed in the pulp and papermaking materials, it is preferred to remove the plastics in advance. When the plastics are admixed in cereals such as soybeans, it is preferable to remove the plastics before processing because the cereals are used for food.
However, there exist no effective means at present for removing plastics admixed in the pulp and papermaking materials or cereals, and at present the plastics dispersed in the pulp and papermaking materials or cereals are found visually and removed manually.